Electrical cable with improved shield

Abstract: A transmission cable (60) for transmitting differential logic signals has shielding (70). The cable includes a pair of insulated signal conductors (62, 64) in side by side relation with a wrap (72) of the shielding including a layer (82) of electrically conductive material wrapped around the two signal conductors. A non-insulated drain wire (66) is disposed axially along the outside of the wrap (72) of shielding adjacent one of the signal conductors (62). The layer (82) of conductive material is continued around the outside of the drain wire (66) thereby forming an additional wrap (74) of the shielding about at least a part of the cable assembly. The drain wire (66) is in electrical engagement with the shielding layer (82).

Description: The present invention relates to an electrical cable having at least two insulated signal conductors and one drain wire in contact with a layer of shielding that is wrapped around the signal conductors along the length of the cable. Modern signal transmission cables typically are shielded by a thin conductive foil and include a drain wire in contact therewith, running the length of the cable, that is used to terminate the foil shield. Such a transmission cable is shown at 10 in Figure 1 of the accompanying drawings. The cable 10 includes a pair of insulated signal conductors 12 and 14 and a non-insulated drain wire 16 all of which are arranged side by side as shown. A layer of conductive shielding material 18 is wrapped around the three conductor assembly so that it is in electrical contact with the non-insulated drain wire. This shielding prevents emissions from the cable as well as providing isolation from nearby or stray signals, and the planar structure of the cable provides advantages in routing and other cable management tasks for certain applications. When this cable is used in differential logic applications with relatively fast rise times and high bit rates, the propagation delay of the signal along the two signal conductors 12 and 14 becomes important. The air gaps 20, as seen in Figure 1, result in asymmetrical capacitive coupling between the shield and the two signal conductors. The dielectric constant is different for each one because the air gaps affect the signal on the conductor 12 more than the signal on the conductor 14, thereby causing different propagation delays for the two signals. In fast switching circuitry, high speed clocklines, and long-run cable configurations this difference can cause the output signal to either not reach the threshold value or, if it does, the signal pulse may be so narrow that it will lack sufficient energy to register as a data bit thereby causing a parity error. A solution to this problem is to arrange the drain wire in the space 22, against the outer insulation of the two signal conductors. However, this adds a bulge in the otherwise flat surface of the cable thereby adversely affecting installation in many applications. Additionally, such an arrangement makes it difficult to terminate the drain wire by automated equipment. Although not concerned with the problem addressed by the present invention, US-A-3 032 604 describes a shielded cable in which a ground wire is disposed against the outer insulation of two signal conductors. US-A-4 800 236 describes a flat cable structure having a corrugated separating member, or septum, disposed between inner and outer sheaths. The septum contacts the sheaths to define tubular envelopes extending along the cable. Each of the envelopes houses one or more insulated conductors and the septum and sheaths are electrically connected to ground potential by drain wires along opposite edges of the septum so that the conductors are electromagnetically isolated along their lengths. What is needed is a transmission cable having signal conductors with substantially similar propagation delays while maintaining the desired flat profile afforded by arranging the drain wire on the same center line as the two signal conductors. To this end the invention consists in a shielded electrical cable comprising two insulated conductors and an uninsulated conductor disposed side-by-side with their axes substantially on the same centre line, said insulated conductors being in abutting relation, and an uninterrupted conductive shielding layer electrically engaging the uninsulated conductor along its length, characterised in that the shielding layer is wrapped completely about the insulated conductors with at least one full turn which separates the insulated conductors from the uninsulated conductor and air gaps adjacent thereto. Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a transmission cable that is known in the industry; Figure 2 is a schematic representation of delay skew in the cable of Figure 1; Figures 3, 4 and 5 schematically represent the output signals resulting from delay skew; Figure 6 is a cross-sectional view of a transmission cable according to one embodiment, illustrating the teachings of the present invention; and Figure 7 is a similar view to that of Figure 6 but shows an alternative embodiment. There is shown in Figure 2 a schematic representation of propagation delay for the pair insulated conductors 12 and 14 of Figure 1, showing, what is known in the industry as "delay skew". The following is a brief discussion of one of the causes of delay skew as it applies to the present invention. A signal is impressed on both signal conductors at the input end 30 of the cable and is shown as a single pulse 32 on each. Note that in differential mode these two pulses would be 180 degrees out of phase, however, for added clarity they are shown in phase. When the signal reaches the output end 34 of the cable, the pulses have shifted to the right, as viewed in Figure 2, an amount equal to the propagation delay for that particular cable type and length. These shifted pulses are identified as 36 and 38. Note that the propagation delay for the conductor 14 is tD1 while the delay for the conductor 12 is a greater amount tD2 caused by the air gaps 20. The delay skew, as known in the industry, is defined as being equal to the absolute value tD2-tD1. The delay skew is further illustrated in Figures 3, 4 and 5. In Figure 3 the differential signal indicated by the pulse forms 40 and 42 are applied to the input end of the conductors 12 and 14. If the output signal were sampled at that point it would look similar to the pulse 44 that peaks well above the threshold voltage 46 and having a full width time duration. If the output signal were sampled at a point significantly further down the length of the cable, the position of the pulse 40 would be retarded with respect to the position of the pulse 42 resulting in significant delay skew. This would result in an output signal similar to the pulse 48 of Figure 4. Note that the width of the portion of the pulse 48 that exceeds the threshold voltage is considerably narrower than that of the pulse 44 of Figure 3. Similarly, if the output signal were sampled much further down the length of the cable, the delay skew would be even greater, resulting in a very narrow pulse width as shown at 50 in Figure 5. While the pulse 50 does exceed the threshold voltage, it is so narrow that it may have insufficient energy to be accepted as a valid data bit. If the delay skew were even greater, the pulse 50 might not exceed the threshold voltage 46, either case resulting in a parity error. By way of example, a typical delay skew for the cable of Figure 1 is about 138 picoseconds per metre (42 picoseconds per foot), resulting in a 4.2 nanosecond delay skew for a cable that is 30.5 metres (100 feet) long. In high frequency applications, such as 500 megahertz and above, the pulse width is only one nanosecond or less so that a 4.2 nanosecond delay skew is completely unworkable. This delay skew can be significantly reduced by shielding the insulated conductor 12 from the effects of the air gaps 20 by placing the shield between the conductor and the air gaps. Such a structure is shown in Figure 6. There, a cable 60 is shown having first and second insulated signal conductors 62 and 64 respectively and a drain wire 66, arranged so that their axes fall on a common plane 68. A layer 70 of shielding is wrapped completely around the two insulated conductors 62 and 64 for at least one full wrap 72, then an additional amount is wrapped about the drain wire 66 as at least a partial wrap 74 and terminated against the full wrap 72 so that the drain wire is sandwiched between the wrap 72 and the wrap 74. The layer 70 of shielding is a composite of two layers, a layer 80 of non-conductive material, such as, polyester or some other suitable carrier material, and a layer 82 of aluminum or other suitable electrically conductive material deposited on the carrier, or otherwise attached thereto. With this arrangement the air gaps 84, adjacent the drain wire 66, are isolated from the insulated signal conductor 62 and, therefore, do not significantly contribute to propagation delay in that conductor. By way of example, a typical delay skew for the cable of Figure 6 is about 16 picoseconds per metre (5 picoseconds per foot), resulting in a 0.5 nanosecond delay skew for a cable that is 30.5 metres (100 feet) long. This is well within the acceptable working range for a 500 megahertz application. The wrap 72 may be multiple wraps around the two insulated conductors and the partial wrap 74 may be a full wrap around the entire assembly or it may be multiple wraps therearound. The only requirement is that the drain wire 66 be disposed between any two adjacent wraps and in electrical engagement with the layer 82 of one of them. In the present example, the non-insulated drain wire 66 is in electrical engagement with the conductive layer 82 of the wrap 74. The conductive layer 82 and the non-conductive layer 80 may be reversed so that the conductive layer is facing outwardly from the wrap 72 so that the drain wire 66 is in electrical engagement therewith instead of with the conductive layer of the wrap 74. An alternative embodiment, as shown in Figure 7, utilizes this reversed layer 70 which is wrapped only around the two insulated signal conductors 62 and 64. The non-insulated drain wire 66 is held in electrical engagement with the conductive layer 82 by means of an outer jacket 90. An important advantage of the present invention is that, in a differential pair cable, significant signal skew is reduced to a negligible amount or completely eliminated while maintaining the drain wire in the same plane as the two signal conductors for ease of cable management. Additionally, by placing the drain wire in the same plane with the signal conductors, it is easier to find and terminate by automated equipment.

Claims: A shielded electrical cable (60) comprising two insulated conductors (62,64) and an uninsulated conductor (66) disposed side-by-side with their axes substantially on the same centre line (68), said insulated conductors being in abutting relation, and an uninterrupted conductive shielding layer (70) electrically engaging the uninsulated conductor along its length, characterised in that the shielding layer (70) is wrapped completely about the insulated conductors (62,64) with at least one full turn (72) which separates the insulated conductors from the uninsulated conductor (66) and air gaps (84) adjacent thereto. A cable as claimed in claim 1, wherein the shielding layer (70) is also at least partially wrapped (74) about the uninsulated conductor (66). A cable as claimed in claim 1, wherein an outer jacket (90) is disposed about the wrapped insulated conductors (62,64) and the uninsulated conductor (66) to hold the latter in electrical engagement with said at least one full turn (72) about the insulated conductors. A cable as claimed in claim 1, 2 or 3, wherein the shielding layer (70) is a composite of two layers comprising a non-conductive layer (80) and a conductive layer (82).

Inventors: ELLIS JOHN RANDOLPH, ELLIS, JOHN RANDOLPH
Applicants: WHITAKER CORP, THE WHITAKER CORPORATION
Application year: 1994
Main classification: H01B11
IPC codes: H01B11, H01B7